(1) Field of the Invention
The present invention relates to a variable polarization plane rotator that rotates the polarization plane of an input light beam. In particular, the invention relates to a variable polarization plane rotator suitable for rotating the polarization plane of linearly polarized light by an arbitrary angle, and an optical device using the same.
(2) Description of Related Art
As a conventional variable polarization plane rotator, there has been known, for example, a variable Faraday rotator that applies a magneto-optical effect in which the polarization plane of an input light beam is rotated by changing a magnetic field to be applied to a magneto-optical crystal. FIG. 22 shows a structural example of a conventional variable Faraday rotator. In FIG. 22, a conventional variable Faraday rotator 100 is arranged on an optical axis between an input side optical fiber 101 having a collimator lens 102 and an output side optical fiber 103 having a collimator lens 104, and rotates the polarization plane of an input light beam output from the collimator lens 102 on the input side by a required angle, to send to the collimator lens 104 on the output side. It is assumed, here, that this variable Faraday rotator 100 is constructed by combining a Faraday element (magneto-optical crystal) 110, with permanent magnets 111 and electromagnets 112 that apply magnetic fields to Faraday element 110 from two directions different from each other by 90 degrees. In such a specific structure, the magnetization direction of the Faraday element 110 points the direction of a composite magnetic field comprising a constant magnetic field from the permanent magnets 111 and a variable magnetic field from the electromagnets 112, and the composite magnetic field is arranged to be strong enough for the magnetization to be saturated. As a result, the magnetization vector of the Faraday element 110 is constant in its magnitude and only its direction is changed. Therefore, the magnetization components parallel to the travel direction of light beam, is changed corresponding to the direction of the composite magnetic field, that is, corresponding to the intensity of the variable magnetic field from the electromagnets 112, and the Faraday rotation angle to be determined by those magnetization components parallel to the travel direction of light beam, is changed corresponding to the intensity of the magnetic field from the electromagnets 112.
Furthermore, for a conventional variable polarization plane rotator, for example, a construction wherein liquid crystal is used has been also known. This conventional variable polarization plane rotator using liquid crystal rotates the polarization plane of an input light beam by changing an electric field applied to the liquid crystal cell.
The conventional variable polarization plane rotators utilizing Faraday elements, liquid crystal and the like as mentioned above are used in various optical devices, for example such as variable optical attenuators, optical switches, optical isolators, optical filters and the like. To be specific, a variable optical attenuator using a variable Faraday rotator is disclosed in Japanese Unexamined Patent Publication No. 6-51255, and an optical filter (gain equalizer) using a variable Faraday rotator is described in Japanese Unexamined Patent Publication No. 11-271700. Furthermore, a variable optical attenuator or an optical switch using liquid crystal is disclosed in Japanese Unexamined Patent Publication Nos. 2001-13477, 11-52339, 7-261140 and 61-285427, Japanese Unexamined Utility Model Publication No. 57-100723, Japanese National Publication Nos. 8-505960 and 8-505961. Moreover, the aforementioned Japanese National Publication No. 8-505961 discloses an optical isolator using liquid crystal.
In various optical devices as mentioned above, adjustment of the attenuation of transmission light, switching of optical paths, control of transmission (loss) wavelength characteristics and the like have been realized based on the rotation control of the polarization plane by a variable polarization plane rotator.
However, for conventional variable polarization plane rotators as mentioned above, in a case of variable Faraday rotators utilizing magneto-optical effects, since the magnetic field should be made variable, electromagnets must be used so that the physical size of the rotator becomes large, and also a comparatively expensive Faraday element is used, so there is a problem of high cost.
Furthermore, in a case of variable polarization plane rotators using liquid crystal, it is easy to change the rotation angle of the polarization plane of an input light beam into either one of two alternative states (for example, a non-rotated state and a state rotated to a specific angle). However, there is a problem in that it is difficult to control the rotation angle of the polarization plane in intermediate states between the two states. In the aforementioned Japanese Unexamined Patent Publication No. 2001-13477, a technique that makes it easy to control intermediate states by arranging two 45° twisted nematic liquid crystals in series is proposed. However, in this case, it is necessary to switch the control of the two twisted nematic liquid crystals according to the rotation angle of the polarization plane. Therefore, there is a disadvantage that the control becomes complicated.